Systems for adjusting the orbit of spacecraft have required the launching of large amounts of propellants, such as gas, from the spacecraft into the space plasma environment to boost or reduce the orbit. Disadvantages with such techniques include the weight and complexities associated with storing the propellants on the spacecraft, with controlling the flow of the propellants, and with operating within the time constraints imposed by a finite propellant supply. Also, to maximize system effectiveness and prevent the unnecessary use of propellants, a separate supplementary system is required to monitor the use and flow of propellants.
A system that can reduce the amount of propellants is one employing an electrodynamic tether. An electrodynamic tether is, in general, a long conducting wire that is deployed from the orbiting spacecraft. The motion of the tether across a magnetic field induces a voltage along the length of the tether. If the system can collect electrons from the space plasma at one end of the tether and expel them back into the space plasma at the other end of the tether, the voltage across the tether can drive a tether current that is capable of adjusting the orbit of the system. One typical device for collecting and expelling electrons is referred to a plasma contactor. This device employs an electrically driven hollow cathode system to ionize and then release an expendable gas such as argon, xenon, or krypton from an onboard gas supply tank. Although the use of a tether reduces the need of propellants, the hollow cathode still requires the use of an expendable resource, (i.e., the gas), which is accompanied by similar disadvantages to those described above with regards to propellants. Thus, there remains a need for a system and method for adjusting the orbit of a spacecraft without the disadvantages of the systems described above.
One objective of the invention is to provide a charge emitter that is suitable for use in space applications, i.e., a charge emitter that is lightweight, small in size, and reliable and robust in the space plasma environment. Another objective is to provide a charge emitter capable of emitting low levels of charge, positive or negative, for controlling the charging of a space object. Another objective is to provide a charge emitter capable of emitting large levels of charge, positive or negative, for use in adjusting the orbit of space objects. Yet another objective is that the charge emitter be capable of emitting charge without requiring high voltages. Still yet another objective is that the charge emitter be capable of emitting electrons without the use of expendable resources.
The invention features a system for altering an orbit of a space object. The system comprises an electrodynamic tether deployed from an orbiting space object and electrically connected so as to be capable of producing electrical charge. A charge-emitting device is located at one end of the electrodynamic tether and has at least two terminals. One of the terminals is electrically connected to the tether to receive the electrical charge produced by the tether. A voltage source applies a voltage across the two terminals of the charge-emitting device to induce the charge-emitting device to emit the charge received from the tether so as to alter the orbit of the space object. The magnitude of the applied voltage can be less than 100 volts. Emission of electrical charge by the charge-emitting device can increase or decrease the orbit of the space object.
The two terminals of the charge-emitting device are a gate and an emitter. In one embodiment, the gate is connected to the tether and power for emitting electrical charge from the tether is supplied by the voltage source applied between the gate and the emitter. In another embodiment, the emitter is connected to the tether and power for emitting electrical charge from the tether is supplied by a source of the electrical charge obtained by the tether.
A second voltage source can apply a voltage between one of the two terminals and the tether to connect that terminal to the tether and to provide a path through the charge-emitting device taken by the electrical charge when drawn from the tether and emitted by the emitting terminal. The path is through the first voltage source when the gate is the terminal connected to the tether. When the emitter is the terminal connected to the tether, the path goes to the emitter without passing through the first voltage source. The voltage applied by the second voltage source can be positive, negative, or zero volts. Zero volts can be achieved by directly connecting the one terminal to the object such as by a wire. The emitted charge can be positive or negative in polarity.
In another aspect, the invention features a method for altering an orbit of a space object. An electrodynamic tether is deployed from an orbiting space object such that the electrodynamic tether is capable of producing electrical charge. A charge-emitting device is placed at one end of the electrodynamic tether. The charge-emitting device has at least two terminals. An electrical charge produced by the tether is received by one of the terminals. A voltage is applied across the two terminals of the charge-emitting device to induce the charge-emitting device to emit the charge received from the tether so as to alter the orbit of the space object. Emission of electrical charge by the charge-emitting device can increase or decrease the orbit of the space object.
The two terminals of the charge-emitting device are a gate and an emitter. When the gate is the terminal electrically connected to the tether, power to emit the electrical charge is from the source that applies the voltage, and a path taken by the emitted charge is through the source of the applied voltage. When the emitter is the terminal that is electrically connected to the tether, power to emit the electrical charge is from a source of the electrical charge on the tether. Here, the path taken by the emitted charge extends to the emitter without passing through the source of the applied voltage.
A second voltage is applied between one of the two terminals and the tether. The second voltage source can be positive, negative, or zero volts. Zero volts can be achieved by directly connecting the one terminal to the tether such as by a wire. The emitted charge can be positive or negative in polarity.